The present invention relates to camshaft assemblies, and more particularly to a molded camshaft assembly for an internal combustion engine.
Small internal combustion engines, such as those used on lawn mowers and snow blowers, generally include a cam gear and at least one cam lobe mounted on a camshaft. The camshaft is rotated by means of the cam gear, which, as is conventional, meshes with a timing gear on a crankshaft to rotate in timed relation to the engine cycle. The cam lobe is used to control an exhaust and an intake valve, as is also conventional.
In the past, such cam lobes have been formed of metallic materials utilizing time consuming and expensive precision machining methods. For example, metallic cam lobes are generally made by utilizing a hobbing machine to cut the cam lobe from a solid piece of metal. Hobbing cam lobes may also result in undesirable high waste or scrap material which further increases their cost of manufacture.
More recently, it has been found that non-metallic cam lobes may be molded from plastics material with sufficient accuracy for small internal combustion engines. Such molded non-metallic cam lobes provide for reduced noise and lower cost compared with metallic cam lobes.
When the camshaft rotates in timed relation to the engine cycle, the cam lobe rotates therewith and acts through cam followers that are positioned to ride on the peripheral surface or profile of the cam lobe and are operatively connected with valves that are cyclically opened when the engine is running to control the intake and exhaust systems of an internal combustion engine. If the follower rides on the edge of the cam lobe, excessive stress and pitting of the peripheral surface of the cam profile may occur. Crowning of the cam profile generally helps to solve this problem. Crowning provides a profile such that the highest point of the cam lobe's peripheral surface which contacts the cam follower is at or near the center of the lobe. In order to crown the cam profile utilizing a conventional molding process, the die tooling typically becomes more expensive and complex with the result that only two cavities may be utilized with such mold compared with four cavities with simpler tooling. More specifically, if the cam mold tooling may be pulled off axially, the tooling is less expensive and complex and four cavities may used in the mold. Normally, however, if axially movable tooling would be utilized to mold a cam lobe, the cam profile cannot be crowned since crowning would normally require the mold tooling to be pulled off radially at 90.degree. to the camshaft axis. As a result, conventional molding techniques would require that a cam lobe have an axially tapered cam profile to allow the tooling to be pulled off axially. Unfortunately, with an axially tapered cam profile the follower would ride on the high edge of the cam profile causing excessive wear and pitting, as noted above.
It is therefore desirable to provide a cam lobe which may be molded of non-metallic materials using a relatively inexpensive molding process which would provide a cam profile that avoids or eliminates excessive wear.